Wireless traffic continues to rapidly increase accompanied by continual increases in demand for frequency, a limited resource. Research related to cognitive radio technology that performs optimal communication by recognizing the local radio environment has been advancing as one way of effectively using frequency. For example, a function that finds frequency white space that corresponds to a time and location such that interference in a system capable of preferentially using frequency does not occur is known as white space (WS) type (or, frequency sharing type) cognitive radio. For example, in the United States of America, use of TV white space (TVWS) communication is under investigation.
With white space type cognitive radio, for example, a system that finds white space and a primary system (or a 1° system), which is a system having privilege to use frequency, is called a secondary system (or a 2° system). In the case of TVWS, a TV broadcast is a primary system.
In a TV broadcast, a wide frequency range is assigned by an ultra-high frequency (UHF) band, etc., whereby not only does the frequency (physical TV channel) used differ according to region, but variations in terms of time are few. Sensing methods and database access methods are present as methods of finding such quasi-static TVWS. For example, even under the rules announced by the Federal Communications Commission (FCC), provisions are given concerning sensing methods and database access methods.
In a database access method, for example, a secondary system accesses a database on a network and obtains WS information that indicates white space. The database, for example, correlates with position information, the WS information that has been calculated using the positions of TV broadcast transmission stations, transmission power, transmission frequency, and other such information. The FCC rules prescribe that a secondary system that uses a database access method is to access a database at least one or more times per day.
On the other hand, according to a known technique for cognitive radio, the frequency to be used by a radio terminal is determined from parameters required for communication, including the available frequency distributed by a database apparatus and the communication distance (for example, refer to Japanese Laid-Open Patent Publication No. 2007-184850).
Nonetheless, with the convention techniques above, a mobile communications apparatus that belongs to a secondary system and has no function for accessing a server that provides WS information cannot determine which local frequency is white space. Consequently, the frequency that the mobile communications apparatus can use for radio communication may be restricted to a frequency that can be used throughout the coverage area of the base station, whereby the utilization efficiency of frequency becomes low.